Anyone built a vault for inground spa?

I'm doing an add-on for hardscape customer. We're expanding patio w/masonry garden wall to include an area for inground hottub. This will be my first one...plan on excavating to desired depth plus enough for concrete base w/drain and sump basin with pump. Surround will match existing masonry garden walls w/slate coping. I'll leave the masonry walls 2' outside width/length of hottub and fill in to edge of spa w/treated lumber grates (like in the bottom of saunas?) for water spillage/rain into the drain/sump area below. Finished height above grade will be around 12". Does that sound about right? This method seems preferable to burying an insulated shell...I'm wondering if anyone has built many of these and can offer some insights that might speed or ease my install? Thanks.

as stated provide ample access for someone to get into the access panel to service the unit plus access to the drains which are usually on the same side of the unit. VERY IMPORTANT - DRAINAGE DRAINAGE DRAINAGE!!!!!!

I've seen many many tubs ruined because the 'vault' as you call it filled with water and fried the pumps. Over kill it

Have you done many of these? I'm leaning toward a poured 4" slab with a center-sloped drain into a sump basin. I'd pipe the basin outside the footprint of the hot tub for easier access. It'll have masonry surround to match our previous garden walls - makes sense to tie base for tub in with footings for garden walls. Thanks.

just remember it's very easy for a drain to get clogged. I have done many of these just never on a slab. I've always done them on a stone base with a pipe to daylight or on dry set pavers. If you wanted to do a slab and had to pour a base for wet walls I would consider just overpouring the base enough to catch the edges of the tub then fill the center of it with stone. Then you have plenty of area for water to exit fast. Just put a drain line in the base of your clean stone fill

I like that idea. Over pour the base for the walls - leave the interior a drywell with center drain. I don't have anywhere to drain the water (from overspill and rain) and planned to let it fill a sump basin w/1.2 h.p pump. I'll leave this outside the vaults footprint. Thanks.